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Glutamine Synthetase: Secondary structures
Glutamine synthetase is composed of 12 identical subunits. Each subunit is composed of 15 alpha helices and 15 beta strands. Each subunit binds 2 Mn for a total of 24 Mn per Glutamine Synthetase.

The β-strands are arranged into 5 beta sheets. In addition, there are 5 β-hairpins and 5 β-bulges. β-bulges are distortions in β-sheets resulting from the addition of an extra residue due to mutation. Their presence allows the proteins to conserve their structure by maintaining the hydrogen bond pattern. At the level of the backbone structure, these β-bulges can cause a simple aneurysm of the β-sheet. Furthermore, each β-bulge can cause a β-sheet to fold over and cross itself.

Within each subunit there are 46 β-turns. These β-turns join secondary structures such as β-sheets and alpha helices when they need to abruptly change directions and usually occur at the protein surface.

Glutamine Synthetase is covalently modified by the addition of Adenosine monophosphate at Tyrosine 397 which is contained in the adenylation loop. In addition, β-loops protrude into the aqueous central channel, allowing for additional quaternary stability.

Each subunit has an exposed NH2 terminus and buried COOH terminus as part of a helical thong, colored in red. The helical thong is used as an anchor inside another subunit.

The active site within the secondary structure can be called a "bifunnel," providing access to ATP and glutamate at opposing ends.

The only ligand present is a pair of Mn ions (Manganese) that indicates the active site of each subunit of the dodecamer.

Glutamine synthetase contains the catalytic sites E327, R339, D50.

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